1. Field of the Invention
The present invention relates to a method of allocating frame offset and link in mobile telecommunications systems (specifically, digital cellular system (DCS) and personal communication service (PCS) system adopting a code division multiple access (CDMA) system). More particularly, the present invention relates to a method of allocating frame offset and link in a base station in which performance of digital telecommunications system is enhanced by allocating a resource in such a manner that loads of the link and frame offset are dispersed when communication channels for use of handoff are allocated, and soft handoff can be occurred when the frame offset is allocated.
2. Description of the Related Art
FIG. 1 illustrates a schematic configuration of a common mobile communications system adopting a CDMA system.
Referring to FIG. 1, the mobile communications system includes a terminal 1 which supports simultaneously or selectively an AMPS (advanced mobile phone service) system and/or CDMA system so that subscribers may communicate via a mobile communication network, a base station 2 which performs communications using a protocol defined in IS-95 in a wireless section together with the terminal 1, a control station 3 which controls wire or wireless link and performs handoff function so as to maintain communications even during the movement of subscribers, an exchanger 4 which constitutes a communication path and performs a connection with other communication network, and a home location register 5 at which subscriber data is recorded and which transmits/receives signals to/from the exchanger 4.
Such a common CDMA mobile communication system is constructed in that the base station 2 efficiently allocate resources, for example, relevant frequency, frame offset, traffic channel, Walsh code, and power gain, for a corresponding call requested by the terminal 1. Then, the base station 2 activates the corresponding traffic channel, the terminal 1 transmits the allocated data (like frame offset or Walsh code) and the allocated resource data to the control station 3, so that the terminal 1, base station 2 and control station 3 may communicate with each other. Such an allocation requires efficient use of the limited frame offset, traffic channel, and link.
In the meantime, the mobile station sends a handoff request message when moved from a current base station to other base station. The control station which has received the handoff request message requests to other base station a set-up of communication channel for handoff. If it is possible to allocate a communication channel for use of handoff using a frequency and frame offset of the message sent from the control station, the other base station to which the communication channel set-up request has been sent allocates Walsh code or power gain managed by the base station to a communication channel, and reports the control station of the success of allocation of communication channel for use of handoff. In case where a frequency and frame offset of the message sent from the control station are absent, failure of allocation of communication channel is reported.
If the frequency strength of other base station is higher than that of the current base station, the mobile station sends a message requesting another handoff. Here, the control station requests other base station to allocate communication channel regardless of frequency or frame offset. Then, the base station allocates communication channel using the frequency and frame offset having the least load. In such case, if the frequency sent from the control station and that sent from the base station are different, hard handoff occurs among frequencies. If the frame offsets are different, hard handoff occurs among frame offsets.
The frame offset among the resources managed by the base station has 16 frame offsets for each link, and a single frame offset is divided into 6 sub-frame offsets, thus 96 sub-frame offsets in total can be used.
Here, the frame offset represents a delay of each frame, and a single frame offset represents a delay of 1.25 msec. To avoid frame overlapping, time for transmitting a frame between the mobile station and the control station is delayed by the amount of allocated frame offset. By this method, data loss between the communication channel and the mobile station may be minimized.
However, in such a conventional method of allocating communication channel for use of handoff, the least-load link is selected when communication channels for use of handoff are allocated in the base station, and the selected link is compared with the frame offset transmitted from the control station. Only in case where the frame offset is unused, the communication channel for use of handoff is allocated. In case where the frame offset transmitted from the control station is used, allocation failure of communication channel for use of handoff is reported to the control station, thus causing handoff failure of the mobile station or inappropriate handoff timing.
In case where the control station requests communication channel allocation for use of handoff regardless of frequency or frame offset, frequency and frame offset received from the control station are not searched. Instead of this, only an effective resource in the base station is searched. As a result, hard handoff among frame offsets which requires acquisition of communication channel for use in handoff of other base station within 0.02 seconds, or hard handoff among frequencies which requires acquisition of communication channel for use in handoff of other base station within 0.08 seconds may occur, thus causing a difficulty in maintaining a call in the mobile station.
Therefore, it is an object of the present invention to provide a method of allocating frame offset and link in a base station in which performance of digital telecommunications system is enhanced by allocating a resource in such a manner that loads of the link and frame offset are dispersed when communication channels for use in handoff are allocated, and soft handoff can be occurred when the frame offset is allocated.
To accomplish the above object of the present invention, there is provided a method of allocating frame offset and link in a base station including: a first step of storing the data transmitted from a control station when a request for allocation of communication channel for use in handoff is received from the control station, and checking a link state; a second step of realigning the links if the links are all normal, comparing a frame offset received from the control station and a frame offset in the realigned link, and, if a link having a frame offset same as that received from the control station exists, determining whether the link is being used or not; a third step of analyzing the method of allocating a communication channel for use in handoff by data transmitted from the control station if it is determined that the link is being used in the second step, selecting a link having less users if the method is analyzed as a free allocation mode in the second step, and selecting the frame offset having the least users in the selected link; a fourth step of allocating the selected link and allocating a resource (frame offset, frequency, Walsh code, and power gain) to a communication channel; and a fifth step of storing the allocated resource to a database and transmitting to the control station a message indicating an allocation success of the communication channel for use in handoff.